Rails Context

Example: Belongs_to is according to the Rails Framework Documentation a public instance method of Module ActiveRecord::Associations::ClassMethods. David A. Black explains in this book, that the instance method ends up as class method of ActiveRecord::Base.

My general (beginner) question is: 1. How do I know in which Rails context I can use which Rails instance methods? Is there any rule of thumb?

2. How is the accessibility of the instance variables accross the MVC? How is it realized Rails internally?

3. Any additional docu on that?

Thanks for Your guidance in advance